Despite advances in understanding many peripheral effects of activating medial olivocochlear (MOC) efferents to outer hair cells (OHCs), as well as the molecular mechanisms underlying those effects, the functional significance of MOC efferents in human audition remains controversial. Basic properties of the MOC reflexes, such as their frequency specificity, are poorly understood. Studies of human efferent function in attention, and detection and/or discrimination of signals, have given seemingly contradictory results, but no study has systematically varied parameters in an effort to resolve these issues. MOC fibers alter the state of the OHCs and thereby affect the generation of otoacoustic emissions (OAEs). Our approach is to use OAEs as a non-invasive assay of MOC activation levels in normal human subjects, and to compare activation levels in a variety of listening contexts. Substantial technical advances allow us to use only a single low-level probe tone, which we can demonstrate evokes little or no MOC activity, to measure MOC activation elicited reflexly by contralateral and/or ipsilateral sounds, or elicited by higher brain centers, and to determine whether these effects are produced by MOC efferents or middle-ear-muscles. To understand the functional organization of the human MOC system, we will determine in humans as functions of probe frequency: (Aim la) the relative strength of the ipsilateral, contralateral and bilateral MOC reflexes, (Aim lb) the frequency specificity of MOC reflexes, and (Aim lc) MOC reflex strength in terms of the changes in cochlear sensitivity produced. To determine if efferents affect cochlear tuning, we will (Aim 2) measure the changes in cochlear tuning produced by efferent activity. To address the functional significance of the MOC system, we will systematically measure MOC activation levels (Aim 3a) in psychophysical tasks in which we predict that MOC activation will aid performance (discrimination of transient signals in noise) and those in which no benefit is anticipated, and (Aim 3b) during selective attention to auditory vs. visual stimuli, or to one component of an auditory stimulus. The results of our experiments will provide the first comprehensive assessment of the functional organization of the human MOC system and should provide significant insight into the functional significance of the MOC system in human audition.